When proguard is enabled on android the app crashes in JNI_OnLoad of libmobileffmpeg.so. The solution for me was to use a heavy hammer and add the line -keep class com.arthenica.mobileffmpeg.** { *; } to my proguard-rules.pro.

It might be the case that there are more exact rules. This issue is to track that and updating any documentation/examples to be proguard friendly.